The invention relates to processes for the cracking of hydrocarbons, particularly for the thermal cracking of a gaseous stream containing hydrocarbons.
In thermal cracking operations a diluent fluid such as steam is usually combined with a hydrocarbon feed such as ethane and/or propane and/or naphtha, and introduced into a cracking furnace. Within the furnace, the feed stream which has been combined with the diluent fluid is converted to a gaseous mixture which primarily contains hydrogen, methane, ethylene, propylene, butadiene, and small amounts of heavier gases. At the furnace exit this mixture is cooled to remove most of the heavier gases, and then compressed. The compressed mixture is routed through various distillation columns where the individual components such as ethylene are purified and separated.
One recognized problem in thermal cracking is the formation of coke. Because coke is a poor thermal conductor, as coke is deposited higher furnace temperatures are required to maintain the gas temperature in the cracking zone at necessary levels. Higher temperatures increase feed consumption and shorten tube life. Also, cracking operations are typically shut down periodically to burn off deposits of coke. This downtime adversely affects production.
Another problem in thermal cracking is the embrittlement of the steel walls in the reaction system. Such embrittlement is due to carburization of the system metallurgy, and ultimately leads to metallurgical failure.
A variety of solutions have been proposed for addressing the problem of coke formation in thermal cracking processes. U.S. Pat. No. 5,015,358 describes certain titanium antifoulants; U.S. Pat. Nos. 4,863,892 and 4,507,196 describe certain antimony and aluminum antifoulants; U.S. Pat. Nos. 4,686,201 and 4,545,893 describe certain antifoulants which are combinations of tin and aluminum, aluminum and antimony, and tin, antimony and aluminum; U.S. Pat. Nos. 4,613,372 and 4,5524,643 describe certain antifoulants which are combinations of tin and copper, antimony and copper, and tin, antimony and copper; U.S. Pat. Nos. 4,666,583 and 4,804,487 describe certain antifoulants which are combinations of gallium and tin, and gallium and antimony; U.S. Pat. No. 4,687,567 describes certain antifoulants which are combinations of indium and tin, and indium and antimony; U.S. Pat. No. 4,692,234 describes certain antifoulants which are combinations of tin and silicon, antimony and silicon, and tin, antimony and silicon; U.S. Pat. No. 4,551,227 describes certain antifoulants which are combinations of tin and phosphorus, phosphorus and antimony, and tin, antimony and phosphorus; U.S. Pat. No. 4,511,405 describes certain tin antifoulants, and antifoulants which are combinations of tin and antimony, germanium and antimony, tin and germanium, and tin, antimony and germanium; U.S. Pat. No. 4,404,087 describes certain tin antifoulants, and antifoulants which are combinations of tin and antimony, germanium and antimony, tin and germanium, and tin, antimony and germanium; and U.S. Pat. No. 4,507,196 describes certain chromium antifoulants, and antifoulants which are combinations of chromium and tin, antimony and chromium, and tin, antimony and chromium.
In King et al, "The Production of Ethylene by the Decomposition of n-Butane; the Prevention of Carbon Formation by the Use of Chromium Plating", Trans. of the E.I.C., 3, #1, 1 (1959), there is described an application of a 3/1000 inch thick chromium plate to a stainless steel reactor. This chromium plate is described as peeling-off the surfaces of the steel after a period of several months of operation, which was attributed to the high temperatures required for the reaction, and periodic heating and cooling.